1. Field of the Invention
The present invention relates to a low temperature method of preparing perovskite-type compounds. More particularly, this invention relates to a low temperature method of preparing compounds such as barium titanate, (BaTiO.sub.3), a piezoelectric material, which is employed (in ceramic form) as an ultra-sound transducer. The method of this invention enables compounds such as barium titanate to be prepared under low temperatures to achieve good phase purity, better homogeneity, and better control of the stoichiometry of the compound.
2. Description of Background Art
Perovskite-type compounds, such as the titanates of the alkaline earth metals occur naturally, but are often too impure to be of much value. Compounds of this type typically have the formula, ABO.sub.3. The compounds are generally synthesized by heating the pure oxides with titanium dioxide below the melting point of the compounds. Barium titanate (BaTiO.sub.3), has five crystalline modifications: cubic, tetragonal, orthorhombic, trigonal, and hexagonal. In particular, the tetragonal form is stable between 5.degree. and 120.degree. C. The tetragonal form is important electrically because when it is exposed to a high DC electrical field, it is permanently polarized. After polarization, it then exhibits two useful properties: it is piezoelectric and ferroelectric. The piezoelectric property leads to applications such as underwater protection equipment, a transducer and ultrasonic equipment, etc. Barium titanate is more convenient than other piezoelectrics such as quartz since it can be readily fabricated in any desired shape. Its high dielectric constant and its temperature coefficient lead additionally to applications in the construction of small capacitors in temperature-sensitive devices that depend upon capacitance.
Multilayer ceramic capacitors are commonly made by casting or otherwise forming insulating layers of dielectric ceramic powder. Powdered metal titanates such as barium titanate etc., can be used as a starting charge to grow crystals. After conversion to single crystal form, these single crystals may then be fabricated at certain orientations which are more efficient than the ceramic counterpart, i.e, a hot pressed sintered powder.
Typically, as set forth in U.S. Pat. Nos. 3,682,766, 3,885,941 and 4,066,426, a base ceramic preparation of dielectric oxides is completely presintered to form a solid solution at high temperature. The material is then ground to a fine powder and mixed with glass frit. A major disadvantage of the prior art methods for making a low temperature fired ceramic capacitors, is the necessity for presintering the base ceramic preparation at a high temperature to form a solid solution of the constitutent oxides before mixing with the glass frit. Another disadvantage of the prior art is the large volume of glass frit necessary to achieve the low firing materials. Though U.S. Pat. No. 4,335,216 attempts to depict a high dielectric constant ceramic composition capable of being fired at temperatures below 1150.degree. C., the operating temperatures show 900.degree. C.-1000.degree. C. Additionally, the ceramic compositions disclosed consist of mixtures of barium titanate, strontium titanate, barium zirconate, titanum dioxide, and manganese dioxide.
There are several other methods which can be utilized in the preparation of compounds such as barium titanate. One such method is disclosed in U.S. Pat. No. 4,485,094 (issued Nov. 27, 1984), which depicts a method for producing continuous mixed oxide films of the general formula MO.sub.n is disclosed, wherein "M" is a mixture of at least two metals and "n" is the number of oxygen atoms in the compound. The two metals, for example, could be barium, strontium, calcium, or mixtures thereof in combination with, for example, titanium, zirconium, hafnium or mixtures thereof. This reference, however, is concerned with the production of metal oxide films at temperatures between 500.degree. C.-600.degree. C.
Barium titanate, a perovskite-type compound, and certain related titanates and zirconates, when properly doped are semiconductive materials which undergo solid phase changes at particular temperatures. It should be noted that when these perovskite-type compounds undergo temperature changes, they may undergo phase changes as well. Therefore, perovskite-type compounds can exist in either a tetragonal, hexagonal, cubic etc. form, depending upon the temperature. Associated with these phase changes are very abrupt and large changes in the resistivity of these materials. The temperature at which such abrupt changes occur are referred to as the curie point and, because the change is positive with respect to temperature, the aforesaid titanates and zirconates are referred to as "positive temperature coefficient" materials. As a result of this temperature anomaly, and the resistance they exhibit, these materials have been found useful as thermosensing elements in numerous diverse applications.
The preparation of barium titanate and related positive temperature coefficient materials involves high temperature ceramic reactions which require careful proportioning of reactants and intermittent intermixture thereof to assure complete reaction and stoichiometric balance in the product. U.S. Pat. No. 3,932,313 (issued Jan. 13, 1976) discloses that an aqueous feed mixture, comprising a compound of barium or other suitable divalent cation, and an organic compound of titanium or zirconium is introduced into an interior zone adjacent a heated reaction zone. The heated reaction zone is a rotating, inclined, tubular furnace or other useful calcining equipment. At the entrance of the reaction zone, the feed mixture forms a mass which swells in response to heat emanating from the tube wall and the reaction zone undergoes dehydration and ignites. Particles break away from the mass and enter the reaction zone where they are calcined in an oxidizing atmosphere to produce the product material.
The careful proportioning of reactants and their intermixture, in processes such as those described above, are factors which are critical to a sharp and definitive response at the curie point. Because the reactants from which barium titanate is produced are solid at the reaction temperature, both the solid state diffusion-controlled reaction and the problem of separating unreacted material from the product render the realization of optimum properties difficult to achieve. In conventional solid state ceramic reaction processes, the raw materials tend to be inadequately mixed and additional contaminants are frequently introduced during mechanical mixing. Also, undesirable crystalline transformations such as anatase (TiO.sub.2) to rutile transformation of titanium dioxide may occur during the process, and the product of the process often fails to possess the desired physical and electrical properties.
U. S. Pat. No. 2,988,422, discloses a process wherein a finely dispersed solution of titanium lactate and a barium compound is passed through a flame for volatilization of the solvent and thermo-decomposition of the barium and titanium compounds to crystalline barium titanate. The product is thus formed as a fine gas-borne dust and therefore process equipment must consequently be adapted for handling substantial volumetric flow rates relative to the rate of production. The fine dust product also presents obvious collection problems and in the case of toxic products such as lead titanate, the operation of this type of process may involve health hazards. Because of its exceptionally small particle size, moreover, the product of such a direct flame reaction process may also be susceptible to discontinuous grain growth. This can result in the formation of large individual grains in the fine grain material.
Though many methods are available for the preparation of compounds such as barium titanate, a low temperature method (less than 500.degree. C.) is nevertheless needed. All of the high temperature methods previously discussed, unfortunately encourage the transportation of impurities from the walls of the apparatus. Additionally, at high temperatures, the reactants experience volatility problems, resulting in less stoichiometric control. Lastly, phase stabilization problems occur at increased temperatures causing the less desirable form of barium titanate, its hexagonal form, to be produced. The tetragonal form of barium titanate is desired, however, due to its unique electrical properties.